Kris forwards an email to 5 people. Each of those people forward that email to 5 more people. If this happens two more times, which expression shows the total number of people who received the email, not including Kris?
A) 4 x 5
B) 1 + 5 + 5^2
C) 5 + 5^2 + 5^3
D) 5 + 5^2 + 5^3 + 5^4